The resonant tunnelling device was originally described by L. L. Chang el al. Appl. Phys. Lett., 24 595 (1974). The conventional resonant tunnelling device is in the form of a diode, although the terminals are often called the `collector` and the `emitter`. Typically the device comprises a quantum well layer (e.g. GaAs) on either side of which is located a respective barrier layer (e.g. AlGaAs).
With this conventional kind of device, application of a bias voltage between the emitter and the collector causes conduction through the layers. At low voltages only a small current flows. However as the bias voltage increases, so does the current. When the energy matches that of a quasi-bound state in the quantum well, electrons can tunnel through the barrier layers so that current freely flows from emitter to collector. At this bias voltage, the quantum well is said to be on resonance, and this value of the bias may be termed the `resonant voltage`.
As the bias voltage is increased beyond the resonant voltage the energy becomes higher than that of the quasi bound state so that tunnelling is inhibited. This gives rise to a region of negative differential resistance above the resonant peak in the IV characteristic. Thus sweeping the collector-emitter bias voltage from a voltage below the resonant voltage to a voltage above the resonant voltage shows a peak in the tunnelling current centred around the resonant voltage, this peak in termed the tunnelling peak.
Since tunnelling is a very fast mechanism of charge transport resonant tunnelling devices offer the potential of extremely high speed operation. They have been described as oscillators (e.g. T. C. L. G Sollner et al. Appl. Phys Lett., 45 1319 (1984)) and switches (e.g. S. K. Diamond et al. Appl. Phys Lett., 54 153 (1989)). Oscillation frequencies up to 712 Ghz have been reported. These devices can be fabricated as far infrared detectors.
Progress in this field has been reviewed in two parts by M. Henini et al. III-V's Review, 7 33 (1994) (Part 1) and III-V's Review, 7 46 (1994) (Part 2).
For production of a good RTD, optimisation of the tunnelling peak characteristics are required. A large difference between the magnitude of the tunnelling current on resonance and off resonance, termed the peak to valley ratio, is required. A fast operating speed also requires a small voltage range over which the device can be switched from the peak current to the valley current.
Previously, most devices of this type have worked on the principle of intraband resonant tunnelling where electrons are injected from the conduction band of the emitter into a conduction band energy state of the quantum well. However, intraband resonant tunnelling devices where electrons are injected from the conduction band into a valence band of the quantum well have also been proposed. In these types of device, resonant tunnelling occurs when the carriers are injected with an energy corresponding to that of a bound state in the valence band. Above the valence band edge lies the band gap. Here there are no states which the carriers can tunnel into. Therefore, once the injected carrier energy is increased above the valance band edge. Tunnelling is suppressed by the band gap.
These devices have been proposed using the antimonide system (J. R. Soderstrom et al. Appl. Phys. Lett 55 1094 (1989)). These devices have been found to exhibit a very low valley current and hence an enhanced Peak to valley ratio. The reason for this is that the electrons are injected into states with an energy below that of the band gap (i.e. the valence band). As the injection energy of the electrons is increased, the energy becomes level with that of the band gap. The electrons cannot tunnel through the band-gap as there are no available states to tunnel into. Hence, tunnelling is effectively suppressed.
An optical device based on the antimonide system is described in U.S. Pat. No. 5,588,015.